Internal combustion engines with high-pressure direct injection are gaining increasingly in importance in engine construction. In common rail injection systems, as they are referred to, the fuel is delivered by means of a pump arrangement from the tank to a fuel rail which serves as a storage reservoir for the fuel. The fuel is already under high pressure in the fuel rail. The fuel can be injected directly into the cylinders via injection valves connected to the rail.
In order to be able to deliver the fuel at high pressure to the fuel rail, the pump arrangement comprises a high-pressure fuel pump. Fuel is fed to this fuel pump via a low-pressure inlet, and the fuel pressure is increased by means of the pump plunger. The fuel then reaches the fuel rail via the high-pressure outlet of the fuel pump.
In order that the necessary pressure can be generated in the inner chamber of the pump by means of upward movement of the pump plunger, the inner chamber of the pump must be separated from the low-pressure side at the start of the pumping process. In state-of-the-art solutions a control valve is provided for this purpose at the low-pressure inlet, which control valve can be closed by means of a valve control signal. It is necessary for this control valve to be closed reliably during the upward movement of the pump plunger so that the fuel pressure needed for the high-pressure direct injection can be built up in the inner chamber of the pump and in the fuel rail connected to the high-pressure outlet of the pump.
As well as the control valve on the input side, a high-pressure pump typically has another nonreturn valve arranged at the high-pressure outlet, which nonreturn valve is designed to prevent fuel flowing from the fuel rail back into the high-pressure pump.
DE 197 08 152 A1 discloses a fuel injection system having a forepump, a high-pressure feed pump and a storage line which is hydraulically connected to the high-pressure feed pump via a nonreturn valve. Injection valves of an internal combustion engine are connected to the storage line. The high-pressure feed pump has an overflow valve which is used for controlling the volume of fuel delivered to the storage line. The high-pressure feed pump has a pump chamber which is limited by a plunger. The plunger is driven via a drive shaft which has multiple cams. The volume of fuel and the fuel pressure under which fuel is delivered to the storage line are set depending on the movement of the plunger and the status of closure of the overflow valve.
In the event of the fuel pressure in the rail being low, for example shortly after the internal combustion engine has started, the nonreturn valve opens at a very early point because the pressure in the inner chamber of the pump exceeds the opposing pressure in the rail at a very early point. It can even happen that the nonreturn valve opens even before the control valve closes. In this case, no sufficiently high fuel pressure can build up in the fuel pump during the upward movement of the pump plunger because the fuel escapes via the nonreturn valve due to the low pressure in the rail. If the valve control pulse for closing the control valve is then emitted, the control valve is not closed or not kept closed as a consequence of the inadequate pressure inside the pump. As a result, fuel also escapes through the control valve back into the low-pressure circuit. The escaping fuel prevents a satisfactory build-up of pressure in the fuel rail. This problem makes itself felt in a negative way particularly when the engine is being started.